Electrochemical fuel cells convert reactants, namely fuel and oxidant fluid streams, to generate electric power and reaction products. Electrochemical fuel cells generally employ an electrolyte disposed between two electrodes, namely a cathode and an anode. An electrocatalyst typically induces the desired electrochemical reactions at the electrodes. In addition to electrocatalyst, the electrodes may also comprise a porous electrically conductive sheet material, or electrode substrate, upon which the electrocatalyst is deposited. The electrocatalyst may be a metal black, an alloy or a supported metal catalyst such as, for example, platinum on carbon.
A particularly interesting fuel cell is the solid polymer electrolyte fuel cell, which employs a membrane electrode assembly (“MEA”). The MEA comprises a solid polymer electrolyte or ion-exchange membrane disposed between the two electrode layers. Flow field plates for directing the reactants across one surface of each electrode substrate are typically disposed on each side of the MEA.
A measure of electrochemical fuel cell performance is the voltage output from the cell for a given current density. Higher performance is associated with a higher voltage output for a given current density or higher current density for a given voltage output.
A direct methanol fuel cell (DMFC) is a type of fuel cell in which methanol is directly oxidized at the anode. Although it may be operated on aqueous methanol vapour, a DMFC generally operates in a liquid feed mode on an aqueous methanol fuel solution. One problem which has been encountered with direct methanol fuel cells is performance degradation, that is, decrease in cell output voltage over time at a given current.
Hamnett et al. (Hamnett, A., Weeks, S. A., Kennedy, B. J, Troughton, G., Christensen, P. A., “Long-Term Poisoning of Methanol Anodes”, Ber. Bunsenges. Phys. Chem. 94, 1014-1020 (1990)) conducted a study of long-term poisoning of methanol anodes. The work was carried out on half cells and not complete fuel cells. Platinum anodes and platinum-ruthenium anodes with a 2.5 M H2SO4 electrolyte and a reference electrode (mercury/mercurous sulphate) were employed.
With respect to pure platinum particle anodes, Hamnett et al. propose that poisoning on the electrode occurs by formation of a place-exchanged oxide which inhibits methanol adsorption. This oxide formation occurs at high anode potentials and can be removed at lower potentials, that is, open circuit. On the other hand, Hamnett et al. show that the amount of oxidised platinum when using a platinum-ruthenium anode is substantially greater that in the pure platinum anode. They further find that the amount of oxidised platinum decreases after extended polarisation and that it appears that the deactivation of platinum-ruthenium anodes is related to a gradual decrease in the amount of oxides on the platinum surface. They conclude that platinum-ruthenium anodes are poisoned by a different mechanism and expect that periodic open-circuiting of the platinum-ruthenium anode would not be so effective in enhancing the lifetime as for platinum anodes and they show test results demonstrating this.
In another study Zelenay et al. (Zelenay, Piotr; Thomas, S. C., Gottesfeld, Shimshon, “Direct Methanol Fuel Cells: Recent Progress In Fuel Efficiency, Cell Performance And Performance Stability”, Electrochemical Society Proceedings, Volume 98-27, 300-315) referring to active DMFC platinum-ruthenium anodes that can be operated for prolonged periods of time without noticeable loss in performance, teach that neither opening of the cell circuit nor stopping the feed of methanol is a prerequisite for stability of anode performance using platinum-ruthenium catalysts.